1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer, a facsimile, a plotter, a multifunctional machine including multiple functions of such devices, etc.
2. Discussion of the Background Art
Currently, image forming apparatuses employing an electro-photographic system are widely spreading in various industrial fields. In such image forming apparatuses, ideally image density is stable under all circumstances and printing conditions avoiding occurrence of abnormal images. However, due to various loads on the toner, such problems sometimes occur. As one of them, when continuous printing of images having a low image area rate is executed, the toner itself or developer including the toner continuously receives stirring load due to small replenishment of fresh toner. As a result, the toner aggregates and roughly accumulates at a gap of a developer lifting section with in the developer unit and impedes lifting thereof, so that abnormal images having white lines or bands are created.
To suppress occurrence of the abnormal images and deterioration of images, in one known approach, when a prescribed condition is met and when a developing roller or sleeve of a developer carrier stops rotation in a first direction during a developing process, the developing roller or sleeve is reversely rotated for a prescribed time period in a second direction opposite to the first direction, so that the aggregated toner can be smashed or removed as discussed in Japanese Patent Application Laid Open Nos. 2004-138675 and 2001-356589. As the prescribed condition, the Japanese Patent Application Laid Open No. 2004-138675 uses one of temperature, a number of printed sheets counted after replacement of a cartridge, a printing rate, and a combination of those. A developer carrier employed in the Japanese Patent Application Laid Open No. 2001-356589 is reversely rotated when a prescribed threshold determined in accordance with one of aforementioned conditions is met.
However, when the printing operation continues for a long time without stopping the developer carrier, the reverse rotation cannot be practiced and the above-mentioned effect cannot be obtained. For example, when an image having a low image area rate is continuously printed for a long time, the opportunity of removing aggregated toner by means of reverse rotation hardly arrives, so that an amount of aggregation increases in proportion to the length of time such printing continues. Further, even though the developer carrier is reversely rotated after a prescribed threshold is exceeded while an image formation device is stopped, the aggregated toner cannot be completely removed.